614 PEOFESSOE STOKES ON THE LONG SPECTEHM OE ELECTEIC LIGHT. 
gibility. On the other hand, the red fluorescence really existing in the early strata, 
when it is overpowered by the blue, may be seen by viewing the crystal through a solu- 
tion of chromate of potash, which greatly enfeebles the blue fluorescence, while at the 
same time it transmits enough of the spectrum to allow the unabsorbed residue to be at 
once distinguishable by its colour (green) from the red fluorescence. In this way the 
red fluorescence may be readily perceived even with electrodes of magnesium. Again, a 
particular stratum which showed a blue fluorescence when acted on by rays which 
entered by a face of the cube, and before reaching it had to traverse some other strata 
showing fluorescence, exhibited a red fluorescence when acted on by rays which fell on 
it directly, having been admitted through an octahedral face. 
It is more difficult to decide as to the identity or diversity of the strata showing 
respectively red fluorescence and blue phosphorescence, because the two efiects are ob- 
served in a diflerent way ; but as far as I could decide, the strata appeared to correspond. 
On the whole, then, I am disposed to think it probable that it is the same substance 
which, in consequence of the action of rays beginning with a part of the violet and ex- 
tending from thence onwards, exhibits a blue fluorescence, which, in consequence of the 
action of rays of extreme refrangibility, exhibits a red fluorescence, and which, in conse- 
quence of the action of rays of a similar refrangibility, exhibits a powerful blue phospho- 
rescence. At least, if the substances be diflerent they would appear to have coexisted in 
solution, and so to have been taken up together in the crystallization of the mineral. I 
should mention, however, that it is contrary to all my experience that the fluorescence 
of a single substance (^. e.‘ not a mixture) should thus, as it were, take a fresh start with a 
totally different colour on proceeding onwards in the spectrum ; but then my experience 
is derived mainly from the examination of substances in the comparatively short solar 
spectrum. — July 1862.] 
I have said that the phosphorescence was produced in certain strata within the 
crystal. These strata were in some places sharply terminated, so as to be foreshortened 
into well-defined lines. On watching the phosphorescence, there was nothing to be seen 
at all like conduction ; the strata remained sharply defined as long as the light was strong 
enough to enable one to judge. This is at variance with one of the two results which, 
on the authority of others, I formerly mentioned as indicating a distinction between 
phosphorescence and fluorescence *. On trying shortly afterwards along with Mr. 
Faraday, I could not obtain either of these results. One of them, that relating to appa- 
rent conduction, which was obtained by MM. Biot and A. C. Becqueeel, has since been 
explained by M. Edmond Becqueeel as an illusion of observation f . The other, that 
relating to the production of phosphorescence in Canton’s phosphorus by rays which 
had traversed a strong solution of bichromate of potash, I am, after a conversation mth 
Dr. Draper, still unable to explain. 
* Philosophical Transactions for 1852, p. 5417. f Annales de Chimie, tom. Iv. (1859) p. 112. 
